Your setting off too fast then. Try running @ 10min/mile pace for your entire run. It's harder than what you think to run at a slower pace for longer. This way you will gain endurance.
I started 9 min/mile.
I train at 8 min/mile
I race @ 7.20-8.00 min/mile
Your heart rate will drop naturally but will go up if your tired.
An hour is a bit long. Try alternating your days, heres mine,
Mon - Rest
Tues - 8 mile (club)
Wed - 5.5 mile (easy)
Thur - 8 mile/speed session/hill session (club)
Fri - Rest
Sat - Rest (or 5.5 mile easy depending on how I feel)
Sun - Long run minimum 10 mile max 15 (one Sunday out of 8 do 18 mile)
